1. Field of the Invention
The present invention generally relates to improvements in linear power amplifier envelope feedback systems, particularly those adapted for use in radio frequency envelope modulation transmitters. More specifically, the present invention relates to envelope feedback loop correction circuitry providing an improved means for reducing power amplifier nonlinearities by maintaining feedback loop balance.
2. Description of the Prior Art
Envelope modulation transmission, i.e. amplitude modulation (AM) and single-sideband (SSB), has been proposed to be a more spectrally efficient modulation scheme than conventional frequency modulation (FM) for radio frequency communications, particularly for land mobile radio applications. There are, however, a number of technical obstacles which must be resolved before this maximum spectrum utilization advantage can be realized. One of the most critical aspects of an envelope modulation system is the reduction of distortion and spurious outputs generated in the transmitter power amplifier. Customarily, transmitters for envelope modulation systems operate as class A or class AB amplifiers in order to achieve linearity, which consequently results in low power efficiency. Although the present invention is not specifically concerned with power efficiency, the proposed solutions to that problem are the basis of the prior art.
One proposed solution to the efficiency problem is the "envelope elimination and restoration" (EER) technique developed by Kahn, in which the SSB signal is initially stripped of its envelope--the remaining phase information being amplified in a conventional class C amplifier--and finally the envelope is remodulated back onto the signal with a conventional amplitude modulator. Even though the resulting waveform can be shown theoretically to be a perfect replication of the original SSB signal, this technique, when implemented, was only moderately successful. The EER transmitter, although quite efficient, was only a few decibels better than linear amplifiers with respect to third-order intermodulation (IM) products, since the high level modulators used for restoring the envelope in the final stage suffered from both nonlinearity of amplitude modulation and spurious phase modulation (AM-to-PM conversion).
The prior work--although directed towards improving efficiency--introduced the concept of resolving generalized modulation on a carrier into amplitude and phase components. In order to improve the spectral purity attainable with the EER approach, Bruene proposed the technique of adding negative feedback in such a way as to cause the envelope of the transmitter output to track the envelope of the AM input signal. FIG. 1 illustrates this approach, wherein envelope feedback system 10 includes input envelope detector 14 which provides an audio frequency (AF) reference to differential amplifier 13, attenuator 15 and output envelope detector 16 which provide an AF feedback to the differential amplifier 13, amplifier 13 produces an envelope error signal to power amplifier control stage 12 which, in turn, instantaneously varies the gain of power amplifier 11 so as to minimize the output distortion. This scheme resulted in a significant improvement in third-order IM products and established the envelope feedback technique as a viable method of reducing both in-band distortion and out-of-band spurious products.
With continued refinement of the envelope feedback technique, it became evident that further improvement in IM performance was limited by spurious phase modulation in the power amplifier. Bradshaw, in British Pat. No. 1,246,209, proposed using negative feedback to correct for this AM-to-PM conversion by implementing a phase shifting network immediately before the amplitude modulator. The phase of the input signal was compared to the phase of the output, and the difference was used to control the phase shifter in such a manner as to reduce phase error. This approach, which has been used in subsequent envelope modulation transmitters, requires the use of a phase-control feedback loop in addition to the envelope-control feedback loop previously discussed.
A significant limitation in envelope tracking accuracy, and ultimately in power amplifier system linearity, is inherent in the two envelope detector concept proposed by Bruene. The two envelope detectors (14 and 16 of FIG. 1) must be precisely matched to prevent mis-tracking between their nonlinear responses. In practice, the two envelope detectors cannot be matched to track closely enough over the entire operating range, thereby causing distortion in the transmitter output. Further improvement in the accuracy of the envelope-control feedback loop concept was realized by Sokol et al. in U.S. Pat. No. 3,900,823, in which a single envelope detector was utilized. FIG. 2 illustrates envelope feedback system 20 employing this technique of subtracting the input RF reference signal from the attenuated output RF feedback signal in RF subtractor 23 and envelope detecting the resulting RF error signal in synchronous envelope detector 22 to provide the aforementioned envelope error signal. The RF subtractor must provide a linear transfer function to realize the improved tracking accuracy, but this is, of course, much more readily achieved than two mutually tracking envelope detectors. Moreover, active synchronous envelope detectors remain linear at low input drive levels, whereas passive envelope detectors, such as diode detectors, do not. This attribute aids in maintaining a higher level of corrective feedback under low drive conditions. It is apparent that the two inputs to envelope comparison mechanism 21 must be in phase for proper system operation; accordingly, this function is accomplished by the supplementary phase-control feedback loop previously described.
A further problem, concerning envelope feedback loop accuracy, arises upon the implementation of envelope comparison mechanism 21 of FIG. 2. Since DC coupling is required in the envelope feedback loop, the DC operating point of the loop must remain very stable. DC bias point stability is achieved through the use of a voltage regulated and temperature compensated system reference voltage. Additionally, the DC drift of the component parts of the loop must be negligible. Unfortunately, this is not the case with commonly available devices. Active synchronous envelope detectors exhibit an unacceptable level of common-mode DC drift due to temperature and voltage variations. Moreover, system balance is also a function of the overall gain of the feedback loop. Therefore, any slight variation in gain of the loop components--due to changes in temperature, varying supply voltage, or aging--will upset the DC balance of the system. Accurate DC balance of the envelope feedback loop in a linear power amplifier system is necessary to achieve acceptable distortion and spurious performance.
A need, therefore, exists for improvement in linear power amplifying systems which maintain the DC balance of the feedback loop under adverse conditions, thereby providing an improved means for reducing power amplifier nonlinearities.